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To get the most value for your dollar, don’t spend more on RAM than your system warrants. If you’re considering upping a 2000MHz or slower system to 256MB or 512MB of memory, instead consider a new PC.
What you have, what you need

The simplest way to ensure you get the right kind of memory is to give the serial number and model name of your computer to the memory supplier, which can be the original system manufacturer, a mail-order or Web-based supplier, or a retail outlet. The supplier should be able to provide the exact memory type you need, and possibly determine compatibility with the RAM modules you have.

To decide how much memory to buy, you need to know how much you’ve got and how much more you can fit. Current RAM is displayed during bootup; you’ll see the memory amount cycle by on the initial boot screen. Other wise, hit F2 or the equivalent key to enter the system BIOS, which should be able to tell you the amount as well.

To see how much memory you can add, open the PC case and take a look at the RAM modules. The amount of memory on each module is usually the total system memory divided by the number of modules. If you have 64GB total and two modules, each contains 32GB. Older systems may have 16MB, 8MB, or even 4MB modules, and you could encounter a mix of them. If your system’s RAM total is an unusual increment, say 40GB or 48GB, you could have such a mix. In that case, pull out all the modules and read their numbers to your supplier. You may have to discard some of the lesser modules to open up enough slots for the upgrade.

Most RAM comes in one of two forms: 72-pin SIMM or 168-pin DIMM. You’ll encounter additional distinctions within these formats, such as FPM DRAM or EDO DRAM (if you’re using SIMMs), or SDRAM (if you’re using DIMMs). Again, rely on your supplier to help ensure that you get compatible types. A recent development is DRDRAM, found on high-speed Pentium-based machines and available in modules called RIMMs.

Another consideration is the rated speed of the memory, measured in nanoseconds. Again, your supplier should be able to help. You must ensure that your new RAM can work with your CPU and the system bus (the so-called front-side bus). SDRAM DIMMs meant for 66MHz operation are usually rated at 10ns, DIMMs for a 100MHz bus are rated at 8ns, and those for a 133MHz bus are rated at 7.5ns.

Installation is easy if you’ve done it before, but it can be tricky the first time. Get assistance from someone who’s installed RAM before if this is your first install. SIMMs are held in place at each end by metal clips and cannot be removed until these are released. The SIMM needs to be inserted into its slot at an angle. As you lift the module toward the vertical, it clicks into place. DIMMs slide straight in. When they’re in position, you lock them in place using two levers, one at either end of the slot.